In case you need a science class reminder, Albert Einstein's Theory of Relativity partly concerned the process of ageing—specifically that a stationary person actually ages faster than someone in motion. Using the iPhone's GPS, along with the Lorentz Transformations, this app will details exactly how much "life" you've won yourself by being active.

The app is a free download, and shouldn't take too long, given it's only a 0.9 MB download. But I suppose you could always gain more time by downloading it while walking around? [iTunes via OhGizmo]